A thin-film transistor (TFT) used in a liquid crystal display device of active matrix type comprises a gate electrode with metal film such as chromium, a gate insulator film made of SiNx, a semiconductor layer made of amorphous silicon, an ohmic contact layer doped with impurities such as phosphor, a source electrode and a drain electrode made of metal film such as chromium, and a protective film laminated in this order on a substrate.
The thin-film transistor as given above is produced by preparing a multiple of thin films on a glass substrate and by performing photo-lithographic process on the thin films. However, for preparing the thin films and for patterning, expensive and low-throughput vacuum systems with complicated structure such as sputtering system, CVD system and etching system must be used. This makes the manufacturing process more complicated and requires higher manufacturing cost.
In this respect, the Patented Reference 1 as given below discloses the manufacture of a thin-film transistor in an atmospheric condition as much as possible. This Patented Reference 1 describes that a gate electrode film of thin-film transistor is produced by ink jet procedure using a liquid material containing a conductive material and to form a source area and a drain area of the thin-film transistor by using a liquid material containing a semiconductor material coated by ink jet procedure.
Also, the Patented Reference 2 describes means to reduce the photo-lithographic process and to simplify the production process, in which a gate bus line and a source bus line are formed on the same layer, and the source bus line being cut off at the crossing portion is bridged in the layer of pixel electrode when the pixel electrode is prepared.    [Patented Reference 1] JP-A-2003-318193    [Patented Reference 2] JP-A-1997-265113
In the manufacture of the thin-film transistor as described in the Patented Reference 1, it is described that the number of the vacuum systems needed is reduced and production processes are simplified by introducing the ink jet procedure. However, many production processes are still required, and it is not possible to manufacture the thin-film transistors at low cost and with high throughput.
According to the method described in the Patented Reference 2, a transparent electrode material with high specific resistance such as indium tin oxide (ITO) (specific resistance: 100 to 1000 μΩ·cm) is used as pixel electrode. As a result, high resistance develops at cross-linking portion to connect the source line.
Normally, a metal material with specific resistance of less than 3 μΩ·cm is used as the bus line. However, even when the length of the blidged portion is designed to be about 5% to 1% of the length of the bus line, the resistance of the bus line is as high as 1.3 to 3 times. It is practically impossible to design the length of the blidged portion to 5% or lower. Further, contact resistance develops between the metal of the bus line with lower specific resistance and ITO with high specific resistance, and this further increases contact resistance at the cross-linking portion.
For large-scale production of high-precision panel for LCD-TV of 100-inch class to be used in the future, the manufacturing method cannot be used, in which wiring resistance is increased even by 1%.
In this respect, it is an object of the present invention to provide a liquid crystal display device using a thin-film transistor and a method for manufacturing the same, by which it is possible to simplify the production process and to reduce the wiring resistance without increasing wiring resistance of the source line and the gate line.